This application claims the priority of German Patent Application No. 100 37 402.6, filed Aug. 1, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a device for treating a fuel/coolant mixture in a fuel cell system.
A fuel cell system with an anode chamber and a cathode chamber is known from the German Patent Document DE 198 07 878 A1. The two chambers are separated from one another by a proton-conducting membrane, oxygen-containing gas being supplied to the cathode chamber and a liquid coolant/fuel mixture being supplied to the anode chamber. The anode chamber forms part of a circuit which also comprises a cooler, a gas separator and a pump. Moreover, this anode circuit of the fuel cell system is divided into a recirculation ring and a cooling ring. Moreover, fuel is passed into the circuit from a tank via a line.
In a further configuration, an ion exchanger is provided in the cooling ring upstream of the gas separator.
Since, in an application of this type, cations and anions are being removed from the coolant/fuel mixture, it is necessary, for example, for a strongly basic ion exchanger to be of correspondingly large design, in order not to become fully laden with the CO2 which is usually entrained in a dissolved fashion in the liquid phase after even a very short operating time. Moreover, the temperatures which prevail in the mixture cause a problem, since commercially available and therefore inexpensive ion exchanger systems are not suitable for the operating temperatures which arise in a fuel cell system of this type. They therefore, in each case, require cooling of the mixture flowing through them before entry into the actual ion exchanger. Therefore, the ion exchanger cannot be positioned directly upstream of the anode chamber, which constitutes a drawback.
A system of similar structure, which likewise describes the treatment of water in a PAFC fuel cell system, is described in U.S. Pat. No. 5,980,716. This system has an ion exchanger device based on a resin ion exchanger and an electro-deionization system. Similar drawbacks to those already described above apply to this system too, but in this case a very complex degassing and filtration system, in which CO2 and iron oxide occurring in the cooling water are removed before reaching the ion exchanger devices, is formed in order to solve the problem.
Moreover, U.S. Pat. No. 5,425,858 and, in a corresponding further development of this document, U.S. Pat. No. 5,954,937 describe a device for the capacitive deionization and electrochemical cleaning of matter flow. The structure of a device of this type includes cells which are stacked on top of one another and through which the flow of matter is passed. Each of the cells has two electrodes with a relatively large surface area. These electrodes are electrically connected in such a manner that a field is produced between them. The charged substances situated in the flow of matter are influenced accordingly by this field and are deflected towards the respective electrode. The flow of matter which leaves the installation has then had electrically conductive substances removed, because these substances remain behind on the individual electrodes.
It is an object of the invention to provide a device for treating a fuel/coolant mixture in a fuel cell system, which ensures that the ion concentration of the fuel/coolant mixture, which is at is at least partially circulating in a circuit system, remains below a level which is critical for a membrane in an anode chamber of a fuel cell of the fuel cell system.
Arranging an additional anion and cation exchanger in the line between the tank and the circuit system has the advantage that, despite the critical operating conditions, conventional ion exchangers can be used in the circuit system. The fuel available in the tank usually includes impurities. These impurities either originate from the production or pass into the fuel during transport or through the infrastructure. According to the invention, the fuel is completely cleaned even before it enters the circuit system. A low temperature level prevails in this line, so that there are no problems with the thermal load-bearing capacity of the ion exchangers. Moreover, the fuel has only very little or no dissolved CO2, which contributes to a very long service life of the anion exchanger. Furthermore, only a small volumetric flow has to be cleaned, so that there are no problems with regard to pressure loss or mechanical durability. Finally, the complete cleaning of the fuel before it enters the circuit system enables a high level of purity to be achieved in the circuit system. Only very minor levels of impurity then occur in the circuit system itself and these can be removed with the aid of a cation exchanger arranged in the circuit system. Since, unlike anion exchangers, conventional cation exchangers are sufficiently able to withstand thermal loads, their positioning in the circuit system is not critical. A further advantage is that only a small ion exchanger is required in the circuit system.
The cation exchanger may either be arranged directly in the circuit system or alternatively in a bypass line. A position in a bypass line may be required in particular if even higher operating temperatures are used in fuel cell systems of the future. If a bypass line is provided, it is additionally also possible to provide an anion exchanger in the bypass line.
To reduce the temperatures in the ion exchanger to acceptable levels, it is moreover also possible for a cooling heat exchanger and/or a heating heat exchanger to be provided upstream or downstream, respectively, of the ion exchanger. In this way, the desired temperature can be maintained in the circuit system and, at the same time, the temperature in the ion exchanger can be reduced.
The fact that the ions have been removed from that part of the fuel/coolant mixture which is flowing in the bypass line and is then combined again with the principal stream of the circuit makes it possible to ensure that the ion concentration in the fuel/coolant mixture flowing in the principal stream is also reduced.
The controllable or regulable valve device enables the ratio of the volumetric flows between the principal stream and the bypass to be adjusted in such a manner that the reduction of the number of ions in the fuel/coolant mixture which is taking place in the bypass is sufficient to ensure that, in the circuit, there is no concentration of ions which could be critical for individual components of the circuit system, in particular a membrane in the anode chamber of the fuel cell. In a preferred configuration, the bypass volumetric flow is controlled or regulated as a function of the temperature of the fuel/coolant mixture.
Moreover, the position in the bypass line offers the advantage that the ion exchanger device can be relatively small, since the volumetric flow passing through it in the region of the bypass line is very low compared to the volumetric flow passing through the circuit as a whole.
The removal of the ionic substances from the fuel/coolant mixture leads to the particular advantage that it is possible to prevent deposits on the membrane which could lead to aging or destruction of the membrane. Moreover, in the presence of certain other substances, for example CO2, the ions may form compounds which could precipitate out and cause corresponding contamination in the fuel cell system.
In an advantageous configuration of the invention, at least one of the ion exchangers is designed as a capacitive deionization system or as an electrodeionization system. These two alternative, advantageous refinements of the invention, as well as allowing the ions to be separated out, also allow at least partial removal of the CO2 which is dissolved in the liquid in the form of carbonic acid. This in turn provides the advantage that the aggressiveness of the mixture, which rises on account of the dissolved CO2 and may cause corrosion phenomena in the region of the fuel cell system, is reduced.
Moreover, reducing the conductive constituents in the fuel/coolant mixture results in further advantages because the conductivity of the mixture is reduced through the removal of the ions and of the dissolved CO2, leading to a fall in the undesirable leakage currents in the fuel cell system.
The ion exchanger in the line between the tank and the circuit system is preferably designed as a mixed bed ion exchanger. However, it is also possible to provide two separate resin bed ion exchangers.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.